The present invention relates to a method for preparing contact lenses containing an ultraviolet-absorbing agent, and capable of absorbing UV radiation, wherein the lens-forming monomer mixture is cured by exposure to UV light.
Lenses such as contact lenses or intraocular lenses may include a UV-absorbing agent in the lens to absorb light in the ultraviolet region of the spectrum, more particularly, to absorb light in the region of about 200 to 400 nm and, especially, about 290 to 400 nm. Representative UV-absorbing materials for such lens applications are described in U.S. Pat. No. 4,304,895 (Loshaek), U.S. Pat. No. 4,528,311 (Beard et al.) and U.S. Pat. No. 4,719,248 (Bambury et al.).
Generally, such lenses are formed by free radical polymerization of a monomer mixture including desired lens-forming monomers, usually in the presence of heat (thermal polymerization) or a light source (photopolymerization). One particular method for producing contact lenses involves thermal polymerization of the initial monomeric mixture in tubes in a heated water bath to provide rod-shaped articles, which rods are then cut into buttons, the buttons then being lathed into contact lenses; such methods for forming lenses including a UV absorbing agent are illustrated in the aforementioned U.S. Pat. No. 4,304,895 (Loshaek) and U.S. Pat. No. 4,528,311 (Beard et al.). Other methods involve casting the lenses directly in molds, wherein the monomer mixture is charged to the mold and polymerized by exposure to ultraviolet radiation.
In the case where it is desired to form lenses by a photopolymerization process, UV curing (i.e., exposure of the monomer mixture to radiation mainly in the ultraviolet region) of the monomer mixtures has proved very effective. It is also possible to effect photopolymerization using a light source also including light in the visible region of the spectrum, although light in this region is generally less efficient in effecting polymerization of conventional lens-forming monomer mixtures than UV curing. However, for lenses including a UV absorbing agent, problems are encountered when attempting to cure the monomer mixtures since this agent absorbs UV light, thus diminishing the amount of UV light available to effect polymerization and resulting in effective or uneven curing of the monomer mixture.
Accordingly, it would be desirable to provide a method whereby lenses exhibiting effective UV-absorbing properties can be polymerized by conventional free radical photopolymerization methods. The present invention provides such a method and solves the aforementioned problems.